The present invention relates to an image forming apparatus of the type developing a latent image formed on an image carrier with a developer, which forms a magnet brush on a developer carrier.
Generally, a copier, printer facsimile apparatus or similar electrophotographic or electrostatic image forming apparatus includes an image carrier implemented as a photoconductive drum or a photoconductive belt. A latent image is formed on the image carrier in accordance with image data. A developing device develops the latent image with toner to thereby produce a corresponding toner image. Today, magnet brush type development using a two-ingredient type developer, i.e., a toner and carrier mixture is predominant over development using a one-ingredient type developer, i.e., toner only. Magnet brush type development is desirable in the aspect of image transfer, reproduction of halftone, stable development against varying temperature and humidity, and so forth. The toner and carrier mixture rises on a developer carrier in the form of brush chains and feeds the toner to a latent image formed on the image carrier in a developing region. The developing region refers to a range over which the magnet brush on the developer carrier contacts the image carrier.
The developer carrier is made up of a sleeve or developing sleeve, which is usually cylindrical, and a magnet roller accommodated in the sleeve. The magnet roller forms an electric field that causes the developer deposited on the sleeve to rise in the form a magnet brush. The carrier of the developer rises on the sleeve in the form of chains along the magnetic lines of force issuing from the magnet roller. The toner, which is charged to preselected polarity, deposits on the carrier forming the chains. The magnet roller has a plurality of magnetic poles each being formed by a particular rod-like or similar magnet. Among the poles, a main pole is positioned on the surface of the sleeve in the developing region for causing the developer to rise. At least one of the sleeve and magnet roller moves relative to the other so as to cause the developer forming the magnet brush on the sleeve to move.
The developer brought to the developing region rises in the form of chains along magnetic lines of force issuing from the main pole of the magnet roller. The chains contact the surface of the image carrier while yielding. The chains feed the toner to the latent image while rubbing themselves against the latent image on the basis of a difference in linear velocity between the developer carrier and the image carrier.
The developer carrier and image carrier are spaced from each other by a preselected development gap at a position where they are closest to each other. When the development gap is increased, the force of the magnet brush rubbing itself against the image carrier decreases. This successfully reduces the omission of the trailing edge of a toner image and faithfully reproduces horizontal lines. However, an increase in development aggravates a so-called edge effect, i.e., increases the amount of toner to deposit on the edges of a latent image, resulting in so-called edge enhancement. Specifically, the edge effect develops solitary dots in a size larger than expected, thickens lines, enhances the contour of a solid image portion and that of a halftone image portion, and causes areas around such image portions to be lost. Consequently, sophisticated control is required over the reproduction of tonality.
By reducing the development gap, it is possible to reduce the edge effect during development and therefore to produce an image with a minimum of granularity. A decrease in development gap, however, intensifies the force of the magnet brush acting on the image carrier. This, coupled with the influence of inverse charge deposited on the carrier, causes the trailing edge of an image to be lost and degrades the reproducibility of horizontal lines and dots. The resulting image is noticeably dependent on direction.
Japanese patent application Nos. 11-39198, 11-128654 and 11-155378, for example, each disclose an image forming apparatus constructed to reduce the omission of the trailing edge of an image even if the image has low contrast. There is, however, an increasing demand for an image forming apparatus capable of implementing further improved image density and image quality.
Technologies relating to the present invention are also disclosed in, e.g., Japanese patent laid-open publication Nos. 8-36303, 10-39620 and 2000-305360 and Japanese Patent 2,941,884.
It is an object of the present invention to provide an image forming apparatus capable of freeing an image from granularity and the omission of a trailing edge.
It is another object of the present invention to provide an image forming apparatus capable of obviating granularity in a halftone or low-density image portion to thereby further enhance image quality.
An image forming apparatus of the present invention develops a latent image formed on an image carrier with a developer that forms a magnet brush on a developer carrier. The developer carrier is made up of a sleeve and a stationary magnet roller accommodated in the sleeve. The magnet roller includes a main pole for causing the developer to form the magnet brush and auxiliary poles for helping the main pole exert a magnetic force. An electric field including an oscillation component is formed between the image carrier and the developer carrier.
A particular ratio is set up between a distance between the image carrier and the developer carrier, as measured at the boundary of a nip, and the shortest distance between them, between the above shortest distance and the shortest distance between the developer carrier and a metering member, or between the shortest distance between the image carrier and the developer carrier and the amount of developer scooped up to the image carrier.